Magnetic read-write head materials in advanced disc drive systems must operate at recording frequencies exceeding 3MHz and must have core widths less than 0.004 inches to assure higher recording densities. The only presently available materials which meet these as well as head fabrication requirements, are magnetic ceramic oxides, specifically nickel-zinc and manganese-zinc ferrites.
Some physical properties of interest in processing of magnetic ceramic materials are porosity and microstructural grain size. Low porosity (high percent of theoretical density) is an important core parameter. When ferrite is glass bonded to form either gaps or core bonding to air-bearing slider, excessive ferrite porosity causes bubble formation in the glass. Subsequent lapping produces pits where glass bubbles were originally located; and since the ferrite is then locally unsupported, this leads to chippage. Ferrite porosity causes loss of dimensional control in gap and apex regions as well as wear problems on air-bearing surfaces. Intra-granular porosity also acts as demagnetization sites thereby reducing permeability. However, small inter-granular porosity is of no consequence.
Microstructural grain size affects several processing parameters. Lapping and polishing rates vary with changes in grain size, so it is desirable to maintain size control to assure material removal rate constancy. Grain size is inversely related to mechanical strength but directly related to permeability.
Nickel-zinc ferrites are limited by their stress sensitivity and their low read signal with thin core widths. With chromium dioxide disc media, pole tip saturation occurs prior to media saturation.
Manganese-zinc ferrite has the highest permeability and saturation induction of the ferrite class of materials and has the advantage of various stoichiometries with nearly zero magneto-crystalline anisotropy and magnetostriction, important for stress insensitivity and low noise. But not until recently has this material been commercially available with the low porosity required for head fabrication operations. Moreover, material processing of manganese-zinc ferrite to achieve low porosity requires pressure-sintering technology. See, for example, U.S. Pat. No. 3,557,266. Pressure-sintering necessitates the use of expensive capital equipment, controls and dies and is a production-limited process when compared with conventional nickel-zinc ferrite processing.
A vacuum-sintering technique which showed that low porosity and high permeability manganese-zinc ferrite at very low frequencies (&lt;0.1 MHz) could be achieved with vacuums of 0.1 to 1.0.mu. followed by a stoichiometric furnace atmosphere soak and then a 1.mu. vacuum cooldown, was published by Shichijo and Takama, in an article entitled "Vacuum-Sintered Mn-Zn Ferrites and Their Properties" in Proceeding of the International Conference on Ferrites, July 1970.
Sintering of manganese-zinc ferrite is complicated by an additional process variable not common to nickel-zinc ferrite sintering, that of furnace atmosphere. In 1955 it was found that manganese ferrite sintered in carbon dioxide had improved magnetic properties over similar ferrites sintered in air. See G. Economos, J.A.C.S., 38, 292 (August 1955). X-ray diffraction analysis of stoichiometric manganese ferrite formulation, in air and nitrogen, has shown the following reactions to occur:
Air Sinter
Mn.sub.3 O.sub.4 + 1/4 O.sub.2 .fwdarw. 11/2 Mn.sub.2 O.sub.3 : 600.degree.-700.degree. C PA0 11/2 mn.sub.2 O.sub.3 .fwdarw. Mn.sub.3 O.sub.4 + 1/4 O.sub.2 : 850.degree.-950.degree. C PA0 Mn.sub.3 O.sub.4 + 3Fe.sub.2 O.sub.3 .fwdarw. 3MnFe.sub.2 O.sub.4 + 1/2 O.sub.2 : .about. 1000.degree. C PA0 Mn.sub.3 O.sub.4 + Fe.sub.2 O.sub.3 .fwdarw. 3MnFe.sub.2 O.sub.4 + 1/2 O.sub.2 : .about. 850.degree. C
Nitrogen Sinter
thus a reducing environment favors manganese-zinc ferrite formation. Moreover, it was also observed that slow cooling in nitrogen rather than air increased induction, i.e. decomposition of mangagese ferrite and subsequent reoxidation, was inhibited.
Manganese-zinc ferrite with a slight excess of Fe.sup.+2 cations are known to produce the highest permeabilities. Excess oxygen partial pressures in the sinter atmosphere will favor Fe.sup.+3 formation while an oxygen deficiency will maintain excess iron cations as Fe.sup.+2. Upon cooling, a thermodynamic tendency exists for Fe.sup.+2 .fwdarw. Fe.sup.+3 so a lower oxygen partial pressure during cool down is required to maintain the Fe.sup.+2 /Fe.sup.+3 ratio achieved during sintering.
However, while sintering atmosphere control assures a high magnetic induction and permeability, it does not necessarily yield a low porosity body. Sintering is a diffusion process which achieves final densification, homogeneous composition, and develops the microstructure. Since diffusion is a thermally activated process, one would expect lower porosities with increasing sinter temperatures. Indeed this occurs in most systems as long as the system is constant, i.e. no loss of a specie nor any phase changes.
Retrograde density with increasing sinter temperature was observed in the BaO-TiO.sub.2 -SiO.sub.2 system due to a phase change. Zinc volatization during ferrite sintering has been reported and limits the upper sinter temperature, oxygen, atmosphere, and vacuum pressure. It has also been shown that at constant sinter temperatures, the lower the oxygen partial pressure, i.e. more reducing atmospheres, the denser the ferrite. Also, lower temperature sintering is characterized by lethargic diffusion rates thereby limiting densification.
From the foregoing, it is apparent that contradictory sintering conditions exist to achieve high porosity manganese-zinc ferrite having good magnetic properties. Consequently, heretofore, the only high quality manganese-zinc ferrites with the requisite magnetic properties have been produced with hot press techniques.